The electrodeposition coating is a sufficiently known process for coating the surface of electrically conducting substrates (compare in this context, for example: Glasurit Handbuch Lacke und Farben, Curt R. Vincentz Verlag, Hanover, 1984, Pages 374 to 384 and pages 457 to 462, and DE-A-35 18 732, DE-A-35 18 770, EP-A-0 040 090, EP-A-0 012 463, EP-A-0 259 181, EP-A-0 433 783 and EP-A-0 262 069). The process is employed to coat metal objects, especially for priming automobile bodies, or else for coating conductive plastics.
The coating materials used in the course of electrodeposition coating generally comprise, as binders, amino- or carboxyl-containing synthetic resins, dispersibility in water being obtained by the neutralization of the amino or carboxyl groups. Special grinding resins, and optionally further, non water-dispersible constituents, such as polymers, plasticizers, pigments, fillers, additives and auxiliaries, are further possible constituents of the electrodeposition coating materials. The crosslinkers employed in the electrodeposition coating materials are either non-dispersible in water or can be water-dispersible, the electrodeposition coating materials being externally crosslinking or else autocrosslinking or being curable by condensation.
Through modification of the binders, selection of the crosslinkers and variation in the composition of the constituents of the electrodeposition coating material, the properties of the coating, for example corrosion protection, adhesion and evenness, are influenced. There are thus known, in particular, electrodeposition coating materials where polymer microparticles or polymer powders--suspended or incorporated by dispersion--are added with the aim of favourably influencing the corrosion protection, especially at edges, the adhesion, especially the impact strength, and the evenness.
Thus it is recommended in EP-A-0 259 181 to overcome the heightened susceptibility to corrosion observed at edges of the coated substrate and caused by an insufficiently thick coating film by adding polymer microgels, the polymer microparticles being characterized by a softening point which is at least 10 degrees C above the bath temperature, by a solubility parameter which differs by not more than 1.0 from the solubility parameter of the depositable synthetic resin, by a refractive index which deviates by 0.02-0.3 from the refractive index of the depositable synthetic resin, and by a crosslinking density of 0.01-5.05 mmol/g, and where, for example, poly(meth)acrylate copolymers in combination with ethylenically unsaturated vinyl compounds can be part of such microgels.
DE-B-26 50 611, EP-A-0 052 831, DE-A-39 40 782, EP-A-0 433 783, SU-A-436890, JP-A-53094346, JP-A-79028410 and JP-A-0624820 describe electrodeposition coating compositions with polymer powders which can be suspended or incorporated by dispersion and which are predominantly free from ionic groups, optionally are able to melt in the course of stoving, and are noncrosslinked or crosslinked, the coating compositions additionally comprising water-dispersible synthetic resins that are typical for electrodeposition coatings. The particle sizes of such polymer powders can in this case markedly exceed the particle sizes of the water-dispersible synthetic resins of known electrodeposition coating materials: the average particle diameter in JP-A-0624820 is from 1 to 50 micrometers, and in DE-A-39 40 782 or EP-A-0 433 783 is from 0.1 to 100 micrometers.
Examples of such pulverulent polymers are polyesters, polyolefins, polystyrene, poly(meth)acrylates, polyurethanes, polyamides, polyvinyl chloride, poly(meth)acrylonitrile, polyoxymethylene, polyvinyl alcohol, butyral resins, ethylene-vinyl acetate copolymers, acrylonitrile-styrene copolymers, acrylonitrile-styrene-butadiene copolymers, polyethylene terephthalate, polybutylene terephthalate, crosslinked urea-aldehyde, triazine-aldehyde and phenol-aldehyde resins, epoxy resins, or cellulose acetate.
In addition, SU-A-661637, SU-A-998592 and SU-A-310952 describe coatings which are formed by cataphoretic deposition of nonaqueous dispersions of polymer powders, the medium used being aliphatic alcohols, such as isopropanol, or tetrachloromethane, and it being possible to add conductive salts, such as ammonium thiocyanate. Polymer powders employed are polyvinyl butyrals.
These coatings are notable for mechanical properties, chemical stability and the properties of adhesion to metallic substrates as known for solvent-containing single-coat systems based on polyvinyl acetal.
The addition to aqueous electrodeposition coating materials of the polymer particles described in EP-A-0 259 181, DE-B-26 50 611, EP-A-0 052 831, EP-A-0 433 783, SU-A-436890, JP-A-53094346, JP-A-79028410 and JP-A-0624820 leads in many cases to improvement in the edge coverage or stone chip protection. In contrast, the corrosion protection afforded by the deposited electrodeposition coating films, especially of the edges, is inadequate despite the improved edge coverage.
Adverse side-effects of the addition of polymer powders are a worsening in the throwing power of the electrode-position coating materials and in the adhesion to the substrate and/or to subsequent coatings, such as coating films applied over them or PVC underbody protection, impairment in the mechanical properties, such as flexibility, extensibility, fracture strength and impact strength, poorer flow properties and a drastic deterioration in evenness.
A disadvantage of the aqueous and nonaqueous formulations described in the patents EP-A-0 25.9 181, DE-B-26 50 611, EP-A-0 052 831, EP-A-0 433 783, SU-A-436890, JP-A-53094346, JP-A-79028410,
JP-A-0624820, SU-A-661637, SU-A-998592 and SU-A-310952 that continues to be important is the inadequate stability of the coating materials, which have a tendency toward sedimentation. In this context, in aqueous electrodeposition coating materials there may be massive covering of the ultrafiltration membranes with coarse polymer particles.